Line Conversions and Bsurfaces with Blender 2.69
I'm not sure about converting back and forth between a grease pencil stroke and a curve, but you can create a surface from grease pencil strokes like this:

Enable Bsurfaces in User preferences &gt; Addons

Create an empty mesh by adding a primitive and deleting all the geometry from edit mode &gt; Delete:


Add a new grease pencil layer in the 3D view &gt; Properties region (N) and select Surface:


Still in edit mode, (editing the empty object) draw grease pencil strokes on the target object with D+LMB:


In the Tool shelf (N in the 3D view), press Add surface in the Bsurfaces panel: